Method for the preparation of aluminized steel sheets to be welded and then press hardened

ABSTRACT

A method for the preparation of steel sheets for fabricating a welded steel blank is provided. The method includes procuring at least two pre-coated steel sheets, each having a pre-coating of an intermetallic alloy layer, topped by a layer of aluminum metal or aluminum alloy or aluminum-based alloy. The sheets have a principal face, an opposite principal face, and at least one secondary face. The sheets are positioned so a gap between 0.02 and 2 mm exists between the secondary faces. The secondary faces face each other. The positioning of the first and second sheets defines a median plane perpendicular to the principal faces. Layers of metal alloy are removed by melting and vaporization simultaneously on the principal faces, in a peripheral zone of the sheets, the peripheral zones being the zones of the principal faces closest in relation to the median plane.

This is a continuation of U.S. patent application Ser. No. 15/306,735,filed Oct. 25, 2016 which is a National Stage of InternationalApplication PCT/162015/000508, filed Apr. 17, 2015 which claims priorityto International Application PCT/162014/000612, filed Apr. 25, 2014, thedisclosures of which are hereby incorporated by reference herein.

The invention relates to a method for the preparation of aluminizedsteel sheets intended to be welded.

The invention further relates to a method for the fabrication of weldedblanks from the aluminized steel sheets described above.

The invention further relates to a method for the fabrication ofpress-hardened parts from the above welded blanks, to be used asstructural or safety parts in automotive vehicles.

BACKGROUND

It is known that welded steel parts can be fabricated from steel blanksthat have different compositions and/or thicknesses, which arecontinuously butt-welded. In one known method of fabrication, thesewelded blanks are cold worked, for example by cold stamping. Accordingto a second known fabrication method, these welded blanks are heated toa temperature that makes possible the austenitization of the steelfollowed by hot forming and rapid cooling in a forming die. Theinvention relates to this second mode of fabrication.

The composition of the steel is selected to make it possible to carryout the heating and hot forming steps and to confer a high mechanicalstrength, high impact strength as well as good resistance to corrosionto the final welded part. Thanks to their ability to absorb impacts,steel parts of this type have applications in particular in theautomobile industry and more particularly for the fabrication ofanti-intrusion parts, structural parts or parts that contribute to thesafety of automotive vehicles.

Among the steels that exhibit the characteristics required for theapplications mentioned above, coated steel sheet as described inpublication EP 971044 includes in particular a pre-coating of analuminum alloy or an aluminum-based alloy. The sheet is coated, forexample by hot dip coating, in a bath having silicon and iron incontrolled quantities, in addition to aluminum. After the hot formingand cooling, it is possible to obtain a mainly martensitemicrostructure, and the mechanical tensile strength can exceed 1500 MPa.

A known method for the fabrication of welded steel parts includesprocuring at least two steel sheets as described in publication EP971044to be butt welded to obtain a welded blank, optionally to cut thiswelded blank, then to heat the welded blank before carrying out a hotforming operation, for example by hot stamping, to confer on the steelpart the form required for its application.

One known welding technique is laser beam welding. This technique hasadvantages in terms of flexibility, quality and productivity compared toother welding techniques such as seam welding or arc welding. However,in the assembly methods that include a melting step, the aluminum-basedpre-coating made up of a layer of intermetallic alloy in contact withthe steel substrate, topped by a metal alloy layer, is diluted duringthe welding operation with the steel substrate within the molten zone,which is the zone that is liquefied during the welding operation andsolidifies after this welding operation, forming the bond between thetwo sheets.

Two phenomena can then occur:

-   -   the first phenomenon is that an increase in the aluminum content        in the molten metal resulting from the dilution of a portion of        the pre-coating in this zone leads to the formation of        intermetallic compounds. These compounds can be sites for the        initiation of cracks when mechanical stress is applied.    -   the second phenomenon is that aluminum, which is an alphagenic        element in solid solution in the molten zone, retards the        transformation into austenite in this zone during the heating        step preceding the hot stamping. It is therefore no longer        possible to obtain a fully tempered structure in the molten zone        after the cooling that follows the hot forming, and the welded        joint includes ferrite. The molten zone then has a hardness and        a mechanical tensile strength less than that of the two adjacent        sheets.

To prevent the first phenomenon described above, publication EP 2007545describes a method that includes removing the superficial layer of metalalloy on the periphery of the sheets to be subjected to the weldingoperation, leaving the intermetallic alloy layer. This removal can becarried out by brushing, machining or by the application of a laserbeam. In the latter case, the width of the removal zone is definedthanks to the longitudinal movement of a laser beam of a certain width,or even by the oscillation of a laser beam smaller than this width,using the edge of the sheet as a reference point. The intermetallicalloy layer is retained to guarantee satisfactory corrosion resistanceand to prevent the phenomena of decarburization and oxidation during theheat treatment that precedes the forming operation.

To prevent the second phenomenon mentioned above, publicationWO2013014512 describes a method that includes, in addition to removingthe metal layer described above, the elimination of the aluminum presenton the cut edge of the sheets before welding, the presence of which, canresult from a cutting operation, and to create a welded joint with afiller metal wire to increase the carbon content of the melted zone inspecific proportions.

BRIEF SUMMARY

In the methods described in the above referenced publications, when theremoval of the layer of metal alloy is the result of a phenomenonincluding a melting, such as a removal by laser beam, there is a more orless significant amount of aluminum that has run over the cut edge(which is also called the secondary face) of the sheet. A subsequentwelding leads to incorporation of this aluminum by dilution in themolten zone and results in welded joints, the mechanical strength and/ortoughness of which are less than those of the base metal.

The different methods for removing the aluminum that has flowed over thecut edge by machining, scraping or ablation by pulsed laser are trickyto carry out due to the difficulty of positioning the blank in relationto the die or to the beam, the rapid wear of the tools when the removalis carried out by mechanical means, or the potential splashing ofaluminum on the prepared faces in the case of a laser ablation on thecut edge.

In addition, after the removal of the layer of aluminum metal on theperiphery of the sheets, the underlying material has a duller and darkerappearance. It is known that laser welding requires a very accuratepositioning of the beam in relation to the plane of the joint formed bythe sheets to be assembled. This positioning and guidance of the beam,or “seam tracking”, is conventionally controlled by sensors that arecapable of detecting the variation, in a direction at a right angle tothe welded joint, of a reflected light beam, whereby the joint planeappears significantly darker. However, the side-to-side placement beforewelding of the two plates from which the metallic layer has been removedfrom the periphery results in only a small variation in contrast at thelevel of the mating plane, which is difficult to detect, and theguidance of the laser beam is then controlled with significantly lessaccuracy.

It is therefore desirable to develop a method for the preparation of theperipheral zones of sheets pre-coated with aluminum that does not havethe disadvantages described above.

It is desirable to have an economical preparation method that makes itpossible to eliminate the expensive, time-consuming and complexoperation of cleaning the aluminum or the aluminum alloy that has runover the secondary face following removal by melting and vaporization.

It is also desirable to have a preparation method that guarantees analuminum content less than 0.3% in the welded joint fabricated fromsheets pre-coated with aluminum or aluminum alloy.

It is also desirable to have a method that improves the accuracy of seamtracking during the welding of sheets pre-coated with aluminum oraluminum alloy, the metal layer of which has been removed over theperiphery.

An object of the present invention is to resolve the problems describedabove.

An object of the invention provides a method for the preparation ofsheets for the fabrication of a welded steel blank including thefollowing steps in succession:

-   -   procurement of at least one pre-coated first steel sheet 11 and        one pre-coated second steel sheet 12, made up of a steel        substrate 25, 26 and a pre-coating 15, 16 made up of an        intermetallic alloy layer 17, 18 in contact with the steel        substrate, topped by a layer of aluminum metal or aluminum alloy        or aluminum-based alloy 19, 20, the first sheet 11 including a        principal face 111, an opposite principal face 112, and at least        one secondary face 71, the second sheet 12 including a principal        face 121, an opposite principal face 122 and at least one        secondary face 72, then    -   the first 11 and second 12 sheets are positioned, leaving a gap        31 of between 0.02 and 2 mm between the secondary faces 71 and        72 facing each other, the positioning of the first 11 and second        12 sheets defining a median plane 51 perpendicular to the        principal faces of the first sheet 11 and the second sheet 12,        then    -   by melting and vaporization simultaneously on the principal        faces 111 and 121, the layer of metal alloy 19 is removed in a        peripheral zone 61 of the sheet 11, and the layer of metal alloy        20 in a peripheral zone 62 of the sheet 12, the peripheral zones        61 and 62 being the zones of the principal faces 111 and 121        closest in relation to the median plane 51 located one on either        side of it.

Preferably, the simultaneous removal by melting and vaporization iscarried out by a laser beam that spans this median plane 51.

The width of the peripheral zone 61 and the width of the peripheral zone62 are preferably between 0.25 and 2.5 mm.

In one particular mode, the width of the peripheral zone 61 and thewidth of the peripheral zone 62 are equal.

In another mode, the width of the peripheral zone 61 and the width ofthe peripheral zone 62 are different.

Preferably, the removal by melting and vaporization occurssimultaneously on the principal faces 111, 121 and 112, 122.

In one particular mode, the metal alloy layers 19, 20 are removed fromthe peripheral zones 61, 62 of each of the first 11 and second 12 steelsheets, leaving their respective intermetallic alloy layers 17, 18 inplace.

In one mode of the invention, the substrates 25, 26 have different steelcompositions.

In one particular mode, the pre-coatings 15, 16 have differentthicknesses.

Advantageously, the metal alloy layer 19, 20 of the pre-coating 15, 16includes, with concentrations expressed by weight, between 8 and 11%silicon, between 2 and 4% iron, the balance of the composition beingaluminum and unavoidable impurities.

The gap 31 between the secondary faces 71 and 72 is advantageouslygreater than 0.04 mm and very advantageously greater than 0.06 mm.

An additional object of the invention provides a method for thefabrication of a welded blank characterized in that at least a first 11and a second 12 sheet prepared by a method according to any of theclaims 1 through 10 is procured, and in that a welding operation of thefirst sheet 11 and second sheet 12 is carried out in the removal zone bymelting and vaporization, along a plane defined by the above-mentionedmedian plane 51, less than one minute after the operation of removal bymelting and vaporization on the first sheet 11 and the second sheet 12.

Preferably, the welding operation is carried out by at least one laserbeam 95.

Preferably, the welding operation is carried out simultaneously by twolaser beams, one of which carries out a welding of the side of theprincipal faces 111 and 121, the other of which carries out a welding ofthe side of the opposite principal faces 112 and 122.

The removal by melting and vaporization is advantageously carried out bya laser beam 80, and the devices that make it possible to carry out theremoval and welding operation are combined in a single piece ofequipment, the relative speed of displacement of which in relation tothe first sheet 11 and the second sheet 12 is identical.

Preferably, the welding operation is carried out by simultaneously usingat least one laser beam 95 and a filler rod 82.

In one particular mode, the removal step is guided by a device thattracks the median plane 51, the coordinates (x-y) defining the locationof the plane 51 at an instant t, are recorded by computerized means, andare used to guide the welding operation that will subsequently takeplace.

In one mode of the invention, the removal step is guided by a firstdevice that tracks the median plane 51, and the welding is guided by asecond device that tracks the median plane and is separate from thefirst device.

In an additional mode of the invention, the sheets 11 and 12 are clampedby a clamping device 98 during the removal operation by melting andvaporization, the clamping being kept constant by the device 98 untilthe welding operation and at least during the welding operation.

An additional object of the invention is a method for the fabrication ofa press-hardened piece from a welded blank including the following stepsin succession:

-   -   at least one welded blank fabricated according to any of the        methods described above is procured, then    -   the welded blank is heated to form, by alloying between the        steel substrate 25, 26 and the pre-coating 15, 16, an        intermetallic alloy compound, in a manner that confers a partly        or totally austenitic structure on the substrate 25, 26, then    -   the welded blank is hot formed to obtain a part, then    -   the part is cooled at a rate sufficient to at least partly form        martensite or bainite in the substrate 25, 26, thereby achieving        a press hardening.

Preferably, the hot forming of the welded blank is carried out by a hotstamping operation.

An additional object of the invention is a welded blank constructed byassembling at least one first 11 and one second 12 pre-coated steelsheet, made up of a steel substrate 25, 26 and a pre-coating 15, 16 madeup of an intermetallic alloy layer 17, 18 in contact with the steelsubstrate, topped by a layer of aluminum metal, aluminum alloy oraluminum-based alloy 19, 20, the first sheet 11 having a principal face111 and an opposite principal face 112, the second sheet 12 having aprincipal face 121 and an opposite principal face 122, the metallicalloy layer 19 being removed by melting and vaporization in a peripheralzone 61 of the sheet 11 and the layer of metal alloy 20 in a peripheralzone 62 from the sheet 12, the welded blank having at least one weldedjoint 52 that defines a median plane 51 perpendicular to the principalfaces of the first sheet 11 and the second sheet 12, and cross sections52 a, 52 b . . . 52 n perpendicular to the median plane 51,characterized in that the morphological characteristics of the layers 17and 18 resulting from solidification after melting and vaporization ofthe pre-coating in the peripheral zones 61 and 62 are identical in thesections 52 a, 52 b, . . . 52 n on either side of the median plane 51.

The sum of the widths of the peripheral zones 61 and 62 preferablyvaries by less than 10% along the welded joint.

Preferably, the layer of metal alloy 19, 20 of the pre-coating 15, 16includes, with concentrations expressed by weight, between 8 and 11%silicon and between 2 and 4% iron, the balance of the composition madeup of aluminum and unavoidable impurities.

An additional object of the invention is a device for the fabrication ofwelded blanks including:

-   -   a device 91 that delivers at least one first 11 and one second        12 steel sheets pre-coated with aluminum, aluminum alloy or an        aluminum-based alloy,    -   a positioning device 92 for the sheets, to obtain a median plane        51 between the sheets 11 and 12,    -   a clamping device 98 for the sheets,    -   at least one source that makes it possible to generate a laser        beam 80 to remove the layer of aluminum metal, aluminum alloy or        aluminum-based alloy by melting and vaporization simultaneously        in a peripheral zone 61, 62 of the first 11 and second 12        sheets,    -   at least one guidance device 94 that makes it possible to        position the laser beam 80 in relation to the median plane 51,    -   at least one source that makes it possible to generate a laser        beam 95 for the welding of the sheets 11 and 12, in the zone        from which the aluminum metal layer 61, 62 has been removed, to        obtain a welded joint,    -   at least one device that makes it possible to obtain a relative        displacement of the sheets 11 and 12 in relation to the laser        beams 80 and 95,    -   the laser beams 80 and 95 being located on a single line with        reference to the median plane 51 and at a fixed distance 64 from        each other.

Preferably, the distance 64 between the laser beams 80 and 95 is between0.5 mm and 2 m. The distance 64 is advantageously less than 600 mm. Inone particular mode, the distance 64 is less than 5 mm.

In one advantageous mode, the laser beam 80 is emitted from an ablationhead and the beam 95 is emitted from a welding head, the heads forming acompact element with a common focusing device for the laser beams 80 and95.

Advantageously, the guidance device 94 also makes it possible toposition the laser beam 95 with reference to the median plane 51.

In one particular mode, the device further includes a filler rod device82 for the construction of the above-mentioned welded joint.

The device advantageously further includes a laser beam that makes itpossible to carry out welding on the face opposite to the face where thebeam 95 operates.

An additional object of the invention is the use of a press-hardenedpart according to the characteristics described above for thefabrication of structural parts, anti-intrusion or impact absorptionparts in vehicles, in particular automobiles.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will becomeapparent from the following description, which is presented by way ofexample and refers to the accompanying figures listed below, in which:

FIG. 1 shows a micrographic section of a sheet precoated with aluminumto be welded, the periphery of which has been prepared according to amethod of the prior art.

FIG. 2 is a schematic diagram illustrating two metal sheets placed endto end, after a treatment according to the invention for thesimultaneous removal of the metallic layer from the periphery.

FIG. 3 illustrates the influence of the positioning gap between twosheets placed end to end, the metal pre-coating layer of which has beenremoved by simultaneous peripheral ablation, on the flow of thepre-coating along the secondary faces of these sheets.

FIG. 4 is a schematic illustration of one preferential embodiment of theinvention.

FIG. 5 is a diagram illustrating a preferred device according to theinvention.

FIG. 6a is a view in elevation of a laser-welded joint constructedaccording to the invention. The micrographs 6 b) and 6 c) show in detailthe surface of two zones where the ablation has taken place that arelocated symmetrically one on either side of the welded joint.

FIG. 7a is a schematic illustration of the steps of the conventionalmethod for the fabrication of welded blanks pre-coated with a metalliccoating. For purposes of comparison, FIG. 7b illustrates the methodaccording to the invention for the fabrication of welded blankspre-coated with a metallic coating.

DETAILED DESCRIPTION

It will be noted that the diagrams do not attempt to reproduce therelative dimensions of the different elements among one another, but areintended merely to facilitate the description of the differentconstituent parts of the invention.

In the methods of the prior art, where the removal of the metallic alloylayer is the result of melting, more or less significant quantities ofaluminum that flow over the secondary face are present. This situationis illustrated in FIG. 1, which shows a macrographic section of a steelsheet 1 mm thick pre-coated with an aluminum alloy from which thesurface metal alloy layer has been removed by melting and vaporizationusing a laser beam. FIG. 1 also illustrates a steel substrate 1 havingan aluminum pre-coating 2 to 25 μm thick. Only one of the two principalfaces of the sheet is illustrated in the macrograph. On the periphery ofone principal face of the sheet, a pulsed laser beam has been used toremove the metallic aluminum layer, leaving the intermetallic layer inplace, thereby creating an ablation zone 3. On account of the vaporpressure or plasma generated by the laser beam, the liquid aluminum isexpelled to the periphery of the zone 3, thereby creating an aluminumaccumulation zone 5. This ablation operation has also created a flow 4of a portion of the aluminum layer over the secondary face, the lengthof which can reach approximately 0.4 mm. Contrary to what happens in thecase of the impact of the laser beam on an organic coating which iscompletely vaporized, the impact of the laser beam on a metallic coatingdoes not result in its total disappearance by vaporization, but in apartial vaporization and melting.

The inventors have shown that this phenomenon of flow along thesecondary face can be prevented by the following method. As illustratedin FIG. 2, at least two sheets 11 and 12 of pre-coated steel areprocured, which can be of the same thickness or different thicknesses.FIG. 2 illustrates the first alternative. At this stage, the sheets 11and 12 are not necessarily rectangular, and the geometry of theircontour is related to the geometry of the final parts that are to befabricated, which will be achieved by a subsequent forming operation.The term “sheet” here is used in a broad sense to mean any objectobtained by cutting from a strip, coil or sheet.

These sheets are made up of a steel substrate 25 and 26, which can be inparticular in the form of a hot-rolled sheet or cold-rolled sheet,depending on the desired thickness. The composition of the substratescan be identical or different, depending on the desired distribution ofthe mechanical characteristics over the final part. These steels areheat-treatable steels, which are capable of undergoing a martensitic orbainitic quenching after an austenitization treatment. The thickness ofthe sheets is preferably between approximately 0.5 and 4 mm, thethickness range used in particular in the fabrication of structural orreinforcement parts for the automobile industry.

The sheets 11 and 12 respectively include principal faces 111, 112 and121, 122. On the surface of each of these faces, there is a pre-coating15 and 16, the thickness and

composition of which can be identical or different in the sheets 1 and2. These pre-coatings 15 and 16 are both obtained by dipping in analuminizing bath.

The pre-coating 15 is itself composed:

of a layer of inter-metallic alloy 17 in contact with the substrate 25.This is an alloy layer of the type Fe_(x)Al_(y) formed by reactionbetween the substrate 25 and the molten metal of the aluminizing bathduring the continuous passage of the sheets through the aluminizingbath. This layer typically has a thickness of 3 to 10 μm. Thealuminizing bath is a bath of aluminum or aluminum alloy in whichaluminum is present in a percentage by weight greater than 50%, or analuminum-based alloy. In this latter case, the aluminum is the maincomponent of the alloy.

of a metallic alloy layer 19, the composition of which is practicallythe same as that of the bath of aluminum, aluminum alloy oraluminum-based alloy.

Likewise, in the sheet 12, the pre-coating 16 is constituted by anintermetallic alloy layer in contact with the substrate 26 and a surfacemetallic layer.

Preferably, the metallic alloy 19, 20 of the pre-coating can containfrom 8 to 11% by weight silicon and from 2 to 4% iron, the balance ofthe composition made up of aluminum and unavoidable impurities. Theaddition of silicon makes it possible in particular to reduce thethickness of the intermetallic layer 17.

The two sheets 11 and 12 can be positioned so that their principal faces111 and 112 are in the same plane 41. In this manner, a laser beamplaced simultaneously over these two sheets will interact with themidentically. However, the two sheets 11 and 12 can also be positionednot exactly in the same plane, i.e. the focal point of the laser beam isnot positioned exactly on the same level in relation to the surface ofthe two sheets with an identical pre-coating. This situation can beencountered, for example, in the case of a difference of thicknessbetween the two sheets 11 and 12. Even in this case, the inventors haveverified that the desired results, in particular the absence of a flowof the pre-coating along the secondary faces, are obtained when themethod is according to the invention is used.

The two sheets 11 and 12 are positioned to place with their secondaryfaces 71 and 72 end to end. This positioning therefore defines a medianplane 51 between the sheets 11 and 12, perpendicular to their principalfaces, and a gap 31 between the sheets.

According to the invention, the respective metallic alloy layers 19 and29 are then removed simultaneously, by a method including a melting anda vaporization, over a peripheral portion 61 of the sheet 11 and aperipheral portion 62 of the sheet 12. As a general rule, the majorityof this removal is due to a melting phenomenon. This rules out methodswhere the layers 19 and 20 are removed purely by vaporization. Thisremoval, which is also called an ablation, is preferably carried out bya pulsed laser beam. The impact of the high power and high energydensity laser on the pre-coating causes a liquefaction and vaporizationof the surface of the latter. On account of the plasma pressure, theliquefied pre-coating is expelled toward the periphery of the zone wherethe ablation is taking place. A succession of short laser pulses withappropriate parameters results in an ablation of the metallic layer 19and 20, leaving the intermetallic alloy layer 17 and 18 in place.However, depending on the degree of corrosion resistance desired on thefinished part, it is also possible to remove a more or less largeportion of the intermetallic layer 17 and 18, for example more than 50%of this layer. The interaction of a pulsed laser beam directed towardthe periphery 61 and 62 of pre-coated sheets, in relative translationwith reference to these sheets, therefore results in a removal of themetallic layer 19 and 20.

The ablation is carried out simultaneously on the sheets 11 and 12, i.e.the means of melting and vaporization are applied simultaneously to theperipheral zones 61 and 62 facing each other. In particular, when theablation is carried out using a laser beam, the laser beam impacts zones61 and 62, spanning the median plane 51. In one preferred mode, a pulsedlaser beam with a rectangular shape is used. A smaller laser beam canalso be used, which is made to oscillate so that it covers the width tobe processed. The method can also be carried out using a principal beamdivided into two rectangular sub-beams, each spanning the median plane51. These two sub-beams can be positioned symmetrically with referenceto the plane 51, or can be longitudinally offset in relation to eachother in the direction of welding. These two sub-beams can be ofidentical or different sizes.

In these different simultaneous ablation modes, it would then beexpected that the aluminum resulting from the melting due to the impactof the laser beam would flow over the secondary phases 71 and 72 underthe influence of gravity and the plasma pressure generated by the beam.

Surprisingly, the inventors have shown that the secondary faces 71 and72 do not experience a flow of aluminum when the gap 31 is between 0.02and 2 mm. Without being bound by a theory, it is thought that thesecondary faces 71 and 72 are covered by a very thin layer of ironand/or aluminum oxide originating from the cutting of the sheets 11 and12. Taking into consideration the interfacial tension between this thinlayer of oxides and liquid aluminum on one hand and the specific gap 31on the other, the surface free of liquid aluminum between the sheets 11and 12 bends to form a wetting angle, without the liquid flowing intothe space 31. A minimal gap of 0.02 mm makes it possible for the beam topass between the sheets 11 and 12, removing potential traces of aluminumthat may have been on the secondary face. Moreover, as will be explainedbelow, in one variant of the method, the welding is carried outimmediately after this ablation operation.

The gap 31 is advantageously greater than 0.04 mm, which makes itpossible to use mechanical cutting methods, the tolerance of which doesnot have to be controlled with extreme precision, which in turn makes itpossible to reduce the costs of production.

In addition, as explained above, the guidance of the welding laser beamis more difficult in the case of sheets from which the coating has beenremoved on the periphery on account of their darker appearance. Theinventors have shown that a width of the gap 31 greater than 0.06 mmmakes it possible to increase significantly the optical contrast of thejoint plane, which appears differentiated in relation to the peripheralablation zones, and therefore ensures that the weld is properlypositioned with respect to the median plane 51.

In addition, the inventors have found that when the gap 31 is greaterthan 2 mm, the mechanism explained above is no longer operative toprevent the flow of liquid aluminum, as the experimental resultsillustrated in FIG. 3 demonstrate.

The gap can advantageously be between 0.02 and 0.2 mm.

For the ablation process, a Q-switched type laser with a nominal powerof several hundred watts can advantageously be used, which deliverspulses of a duration on the order of 1/50 of a nanosecond with a maximumpower of 1-20 megawatts. This type of laser makes it possible, forexample, to obtain an impact zone of the rectangular beam of 2 mm (in adirection perpendicular to the median plane 51) and 1 mm, or less than 1mm (for example 0.5 mm) in the direction of the length of this medianplane. The displacement of the beam then makes it possible to createablation zones 61 and 62 on either side of the plane 51 without theoccurrence of a flow along the faces 71 and 72.

The morphology of the ablation zones 61 and 62 will naturally be adaptedto the welding conditions that follow, in particular to the width of thewelded zone. It is thus possible, depending on the nature and the powerof the welding process that will follow, for the width of each of theablation zones 61 and 62 to be between 0.25 and 2.5 mm or, for example,in the case of hybrid laser arc or plasma welding, between 0.25 and 3mm. The ablation conditions will be selected so that the sum of thewidths of the ablation zones 61 and 62 is greater than the width of thewelded zone.

If the sheets 11 and 12 are identical, it is possible to specify thatthe widths of the ablation zones 61 and 62 are also identical. But it isalso possible to specify, for example using a horizontal shift of thelaser beam in the lateral direction with reference to the median plane51, for the widths of these ablation zones to be different.

According to the invention, the ablation can be carried out only oneside of the principal faces. FIG. 2 therefore illustrates this case,where the simultaneous peripheral ablation has been carried out only onthe side of the principal faces 111 and 121.

However, to minimize, as far as possible, the introduction of aluminumduring welding to be carried out on the sheets, it is also possible topreferably carry out this simultaneous peripheral ablation on all of thefaces, i.e. 111, 121, 112, 122. For this purpose, in the case of anablation by laser welding, a device of the “power switch” type can be

advantageously used, which divides the power of the beam, one part beingused for the simultaneous ablation of the phases 111 and 121, and theother part for the simultaneous ablation of the faces 112 a 122. It isalso possible to use a second laser which is separate from the first.

After this simultaneous ablation operation, there will be two sheets,from the periphery of which the metallic alloy layer has been removed,that are suitable for welding. This welding can be done later and thesheets can be either kept facing each other or separated. They can beseparated easily because the method according to the invention makes itpossible to limit the flow of liquid aluminum between the sheets so thata solidified flow does not create any undesirable mechanical bond.

But the inventors have also discovered that an in-line welding operationcan be advantageously carried out on the sheets prepared in the mannerdescribed above. On account of the absence of a flow of aluminum overthe secondary face, the prepared sheets can be welded immediatelywithout the need to remove the sheets from the line and then repositionthem after cleaning. The interval of time that elapses between thesimultaneous ablation operation and the welding operation is less thanone minute, which minimizes oxidation on the faces 71 and 72 andachieves higher productivity. In addition, when this interval of time isshort, the welding is done on the sheets that have been preheated by theablation operation so that the quantity of energy to be applied for thewelding can be reduced.

It is also possible to use any continuous welding method appropriate tothe thicknesses and productivity and quality conditions required for thewelded joints, and in particular:

-   -   laser beam welding    -   electric arc welding, in particular using the TIG (“Tungsten        Inert Gas”), plasma, MIG (“Metal Inert Gas”) or MAG (“Metal        Active Gas”) methods    -   electron beam welding.

Laser welding is one method that can be used advantageously on accountof the high energy density inherent to this method, which makes itpossible to obtain a narrow molten zone which varies within smallproportions. This method can be used by itself or in combination with afiller rod 82, as illustrated in FIG. 5. In this case, it is possible tomodify the composition of the molten zone thanks to a composition of thefiller rod which is different from that of the compositions of thesheets 25 and 26. The welding method that combines a laser beam and afiller rod can then include either a method where the filler rod ismelted only by the laser beam or by a hybrid laser-TIG welding method,i.e. a laser beam combined with an electric arc delivered by a TIGwelding torch equipped with a non-melting electrode, or a hybridlaser-MIG welding method in which the welding torch is equipped with amelting electrode rod.

According to one variant of the invention, the devices that carry outthe simultaneous ablation and welding operations are combined into asingle piece of equipment. This equipment is driven at a single rate ofrelative displacement with reference to the sheets. In this equipment,the rate of simultaneous ablation is identical to the welding speed,which makes it possible to conduct fabrication under optimal conditionsof productivity and efficiency.

FIG. 4 illustrates one preferential variant of the invention. The figureshows the sheets 11 and 12 having a pre-coating of aluminum, aluminumalloy or aluminum-based alloy. A first laser beam 80 carries out asimultaneous ablation of a peripheral zone 61 of the sheet 11 and aperipheral zone 62 of the sheet 12, whereby the laser beam spans themedian plane of the sheets 11 and 12. A second laser beam 81simultaneously carries out an identical operation on the bottom face ofthe sheet. In one variant (not illustrated in FIG. 4), only one laserbeam 80 carries out the ablation, which is not carried out on theopposite face. This variant will be used when it is not necessary toachieve a very low aluminum content in the welded zone that willsubsequently be created.

At a certain distance 64 from this first ablation zone, a laser beam 95carries out the welding of the sheets 11 and 12 to create a welded zone63. The distance between the ablation and welding devices is keptconstant using a device, which is itself known and is representedschematically as 96. The sheets 11 and 12 are displaced with referenceto this assembly 96 along to the path indicated by 97.

The sheets 11 and 12 are advantageously clamped using a clamping device,which is not shown in FIG. 4. The sheets are clamped during the ablationoperation by the beams 80 and 81. This clamping is maintained throughthe welding process, which is itself carried out by the beam 95. In thismanner, no relative displacement occurs between the sheets 11 and 12 andthe welding by the laser beam 95 can be carried out with greaterprecision.

The maximum distance between the points of impact of the beams 80, 81 onone hand and 95 on the other hand depends in particular on the weldingspeed. As described above, the welding speed will be determined inparticular so that the time that elapses between the impacts of thebeams (80, 81) and 95 is less than one minute. This maximum distance canbe preferably less than 2 m so that the equipment is particularlycompact.

The minimum distance 64 between these points of impact can be reduced to0.5 mm. A distance smaller than 0.5 mm would result in an undesirableinteraction between the ablation beams 80, 81 on one hand and the“keyhole” which is inherently present during welding by the beam 95 onthe other hand.

A short distance 64 can also be obtained by combining the two ablationand welding heads (the heads being defined as the devices from which thelaser beams are emitted) into a single more compact head, whereby thelatter can use, for example, the same focusing element for the ablationand welding operation.

A very small distance 64 makes it possible to implement the method usinga particularly compact unit and to proceed so that a certain quantity ofthe thermal energy delivered by the laser beams 80 and 81 is added tothe linear welding energy delivered by the beam 95, thereby increasingthe total energy efficiency of the method. A very small distance makesit possible to shorten the cycle time necessary for the unit productionof a welded blank, and thus to increase productivity. These effects areobtained in particular when the distance 64 is less than 600 mm or evenless than 5 mm.

FIG. 5 is a schematic diagram of a preferred device according to theinvention. This device includes the following elements:

-   -   a station A having a delivery device 91 which is known in        itself, which makes it possible to deliver at least one first 11        and one second 12 steel sheet pre-coated with aluminum or        aluminum alloy or an aluminum-based alloy.    -   a station B having a positioning device 92 for these sheets 11        and 12, which is itself also known. After the positioning of the        sheets, a virtual median plane 51 is therefore defined.    -   a station C having a clamping device 98 for these sheets 11 and        12, which is itself known, and which can be, for example, a        magnetic, mechanical or hydraulic clamping device.    -   a station D having at least one guidance device 94 which is        itself known, and makes it possible to detect the median plane        51 and to position the laser beam 80 with reference to this        median plane. This device can include, for example, an        illumination of the zone of the median plane by a light beam,        and a photosensitive CCD or CMOS sensor for the reflected beam,        which makes it possible to locate the position (x, y) of the        median plane at a given instant. This makes it possible to        control the positioning of the ablation laser beam 80, which is        downstream in the relative welding direction, so that its        position coincides with the desired location of the ablation        zone.    -   at least one source that makes it possible to obtain a laser        beam 80 to remove by melting and vaporization the metallic        aluminum layer simultaneously on the peripheral zone on either        side of the median plane 51. As mentioned above, a second laser        beam 81 (not illustrated in FIG. 5) can also carry out the same        operation on the opposite faces.    -   at least one source that makes it possible to obtain a laser        beam 95 welding of the sheets 11 and 12 in the zone from which        the metallic aluminum layer 61, 62 is removed, to obtain a        welded joint. The laser source used can be selected from among a        laser source of the CO₂ gas laser type with a wavelength of 10        μm or a solid-state laser source wavelength of 1 μm. Taking into        consideration the thickness of the sheets, which is typically        less than 3 mm, the power of the CO₂ gas laser will be greater        than or equal to 3, or even to 7 kW; in the case of a        solid-state laser, the power will be greater than or equal to 2,        or even 4 kW.

Optionally, a second laser beam, of a type similar to 95, can be appliedin the lower portion, i.e. on the opposite face. This arrangement makesit possible to increase the welding speed and/or to reduce the unitpower of the source 95.

This beam 95 can be guided either by its own guidance device which isseparate from 94 (case not illustrated in FIG. 5) or by the device 94.The inventors have discovered that this latter solution is particularlyadvantageous because it makes it possible for the weld to be positionedexactly in the zone where the ablation has been carried out, i.e. forthe two steps of ablation and welding to be completely coordinated.

Optionally, the assembly can include a filler rod device 82 to modifythe composition of the molten zone thanks to a composition of the fillerrod that is different from the compositions of the sheets 25 and 26.

The sheets 11 and 12 are moved from the station A toward the station Dto obtain a relative displacement of the sheets with reference to thelaser beams 80 and 95, the latter being positioned on the same line withreference to the median plane 51, and at a fixed distance 64 from eachother.

As noted above, this distance 64 is preferably between 0.5 mm and 2 m,preferably between 0.5 mm and 600 mm, or between 0.5 mm and 5 mm.

The welded blank obtained by the method according to the invention hasthe following specific characteristics:

-   -   as illustrated in FIG. 5, the welding of the blanks takes place        in line along the median plane 51 on the blanks 11 and 12 that        have undergone a simultaneous ablation by the beam 95. The        ablation results in a melting and vaporization of the coating,        wherein its subsequent solidification occurs forming specific        ripples, the spacing of which is a function of the pulse        duration and the advancing speed of the ablation beam. In the        method illustrated in FIG. 5, this solidification morphology is        identical on both sides of the plane 51, because the ablation is        carried out using a beam that spans this joint plane. FIG. 6        also shows a macrographic view in elevation of a welded joint        created using the method illustrated in FIG. 5. The zones 13 and        14, which have undergone a simultaneous ablation, are located        one on either side of the weld 23. If we consider the zones 21        and 22 located facing each other along a cross section 52 a, we        find that the solidification morphology is identical. The same        is true for the other sections 52 b . . . 52 n. In addition,        when the welding laser beam 95 impacts the two sheets to be        assembled, this impact occurs over zones, the reflectivity of        which is identical on either side of the plane 51, so that an        absolutely identical depth of penetration is obtained on both        sides of this plane. The invention therefore makes it possible        to obtain a very regular geometry of the final welded joint and        a very uniform dilution of the aluminum in the weld, regardless        of the cross-section 52 a, 52 b . . . 52 n in question.    -   on the other hand, in the prior art, it has been seen that the        ablation was carried out on only one sheet at a time, using a        laser beam in longitudinal displacement that uses the edge of        the sheet as a reference point. However, in spite of the        precautions taken in the operation of cutting the sheets, the        straightness of a prepared edge inevitably includes some        variation with reference to an ideal straight line, whereby the        variation can be quantified by a standard deviation σ₁. In        addition, the longitudinal displacement of the laser beam is        itself subject to a variation of its position in the transverse        direction, which can be quantified by a standard deviation σ₂.        This method therefore produces a sheet, the width of the        ablation zone of which having a standard deviation variation        (σ₁+σ₂) in the longitudinal direction of the ablation operation.        After this operation, these two sheets are placed next to one        another and then welded. The result is a welded blank, the total        width of the ablation zone of which includes a certain        variability, which is the sum of the variabilities associated        with each of the two sheets, i.e. 2(σ₁+σ₂).    -   for purposes of comparison, in the method according to the        invention, the ablation is carried out by using a single        reference plane, the median plane 51, and the ablation operation        is carried out in a single step so that the variability of the        total width of the zone with ablation in the longitudinal        direction is equal to (σ₁+σ₂), i.e. a reduction by one-half        compared to the prior art. Width measurements of the total        ablation zone taken at different positions along a welded joint        show that the width varies by less than 10%.

In summary, FIGS. 7a and b schematically illustrate the comparison ofthe steps of the conventional method for the fabrication of weldedblanks pre-coated with a metallic coating with the method according tothe invention.

In the case of the conventional method (FIG. 7a ), the ablation of themetallic pre-coating is carried out on the periphery of each sheet,wherein this operation is conducted individually on each sheet (stepA1). Then (step A2), the pre-coating that has run over the cut edgeresulting from step A1 is removed. After an intermediate storage of thesheets (step A3), the sheets are positioned for their assembly bywelding (Step A4). After this positioning, there is no symmetry betweenthe solidification structures in the peripheral ablation zones, wherebythese structures are positioned randomly with reference to the medianmating plane of the sheets. The sheets are then welded (step A5).

In the case of the method according to the invention (FIG. 7b ), themetallic pre-coating on the periphery of the sheets placed end to end isremoved simultaneously, maintaining a specific gap between the sheets(step B1). This operation produces a situation in which thesolidification structures are identical and symmetrical on either sideof the median positioning plane. Then, without an intermediate step, thesheets thus prepared are immediately assembled (step B2).

It is therefore apparent that the welded joints constructed according tothe conventional method and according to the invention differ in termsof the morphological characteristics in the solidification zones inimmediate proximity to the molten metal created by welding.

By way of non-limiting examples, the following embodiments illustratethe advantages achieved by the invention.

EXAMPLE

Steel sheets 1.2 mm thick having the following composition by weight:0.23% C, 1.19% Mn, 0.014% P, 0.001% S, 0.27% Si, 0.028% Al, 0.034% Ti,0.003% B and 0.18% Cr, with the balance made up of iron and impuritiesresulting from processing, are procured. These blanks include apre-coating 30 μm thick on each face. This pre-coating is made up of anintermetallic layer 5 μm thick in contact with the steel substratecontaining 50% by weight aluminum, 40% by weight iron and 10% by weightsilicon. This intermetallic alloy layer results from the reactionbetween the steel substrate and the aluminum alloy bath.

The intermetallic layer is topped by a metallic layer 25 μm thick,containing, by weight, 9% silicon, 3% iron and the balance made up ofaluminum and unavoidable impurities.

The dimensions of these sheets are 400 mm×800 mm. The welding is to becarried out on the edges 400 mm long.

Two of these sheets are positioned so that the gap between their facingedges is 0.1 mm. The metallic layer on the periphery of these sheets isthen removed using a pulsed laser with an average power of 800 W.

This ablation is carried out simultaneously by two beams on each of theopposite faces of the sheet. The sheets are placed in motion withreference to the beam at a constant speed V=6 m/min. Each of the beamsis focused to obtain a rectangular focal spot 2 mm×0.5 mm, the distanceof 2 mm extending in the transverse direction with reference to themedian plane of the two sheets. In this manner, two sheets are createdsimultaneously, from the periphery of which the metallic layer isremoved over a width of practically 1 mm on each of the sheets. Thisablation operation is guided by a sensor that detects the position ofthe median plane between the two sheets, located immediately upstreamwith reference to the two pulsed ablation laser beams, in a positionidentified as x₀. The sensor is located at a distance d₁ approximately100 mm from the ablation beams. At the level of the sensor, thecoordinates (x₀, y₀) of the position of the median plane are recorded atan instant t₀ by computerized means. As the sheets move at a speed v,this plane position reaches the level of the pulsed ablation beams at aninstant

$t_{1} = {t_{0} + {\frac{d_{1}}{V}.}}$

Thanks to a guidance device of the laser beams, the exact position ofthe impact of the laser beams on the sheets that occurs at the instantt₁ is adapted so that it corresponds exactly to the ablation zonedefined on the basis of the position of the median plane.

After ablation, a laser beam located at a fixed distance d₂ of 200 mmfrom the pulsed laser beams makes it possible to create a welded jointbetween these sheets. The welding is carried out with a linear power of0.6 kJ/cm, under the protection of helium, to prevent decarburization,oxidation and hydrogen absorption phenomena. The length of time thatelapses between the ablation operation and the welding is 2 seconds.

The welding laser beam is guided here again using the sensor locatedupstream of the ablation operation. The position of the median planerecorded at the instant t₀ arrives at the level of the welding laserbeam at the instant

$t_{2} = {t_{0} + {\frac{\left( {d_{1} + d_{2}} \right)}{V}.}}$

The precise position of the impact of the welding laser beam is thenadjusted using the optical guidance device of the laser beam, so that itis centered on the position of the previously defined median plane.

FIG. 6a is a macrograph illustrating a view in elevation of thelaser-welded joint obtained, in which the weld 23 is surrounded by twozones 13 and 14 where the ablation has been carried out simultaneously.The total ablation width 24 is an average of 1.92 mm and varies by lessthan 10% over the length of the welded blank.

FIGS. 6b and 6c are enlarged views of the surface of zones 21 and 22,which are located symmetrically on either side of the section 52 atransverse to the welded joint. It has been found that thesolidification wrinkles of these zones 21 and 22 are identical on eitherside of the welded joint and have a continuous character.

In addition, a Castaing microprobe was used to analyze the aluminumcontent of the welded zone thus created. The aluminum content remainsbelow 0.3%, which clearly indicates that the quantity of aluminum on thesecondary faces, after the ablation step and before welding, ispractically zero.

A welded blank assembled under the conditions of the invention was thenheated in a furnace to a temperature of 900° C. and held at thistemperature, whereby the total hold time in the furnace was 6 minutes.The heated blank was then hot stamped to form a part, which was held inthe stamping press tool to cool the part at a rate greater than thecritical martensitic tempering rate of the steel.

It was then found that the welded zone on the hot stamped piece did notcontain any brittle Fe—Al intermetallic compounds, and that the hardnessof the melted zone was practically identical to that of the base metal.

The invention therefore makes it possible to economically producestructural and safety parts for the automobile industry from aluminizedsheets having a welded joint.

What is claimed is:
 1. A method for preparation of steel sheets to beused for fabrication of a welded steel blank, the method comprising thesteps of: providing a pre-coated steel first sheet having a first steelsubstrate and a first pre-coating including a first intermetallic alloylayer in contact with the first steel substrate, topped by a first metalalloy layer of aluminum metal or aluminum alloy or aluminum-based alloy,the first sheet comprising a first principal face, a first oppositeprincipal face and a first secondary face; providing a pre-coated steelsecond sheet having a second steel substrate and a second pre-coatingincluding a second intermetallic alloy layer in contact with the secondsteel substrate, topped by a second metal alloy layer of aluminum metalor aluminum alloy or aluminum-based alloy, the second sheet comprising asecond principal face, a second opposite principal face and a secondsecondary face; positioning the first and second sheets so that thefirst and second secondary faces face each other and define a medianplane perpendicular to the first and second principal faces of thefirst; and simultaneously melting and vaporizing the first and secondmetal alloy layers in first and second peripheral zones of the first andsecond sheets, respectively, on the first and second principal facesclosest to the median plane.
 2. The method as recited in claim 1wherein, during the positioning step, the first and second sheets arepositioned such that a gap between the first and second secondary facesis less than or equal to 2 mm.
 3. The method as recited in claim 1wherein the simultaneous melting and vaporizing is carried out by alaser beam spanning the median plane.
 4. The method as recited in claim1 wherein widths of the first and second peripheral zones are from 0.25to 2.5 mm.
 5. The method as recited in claim 1 wherein widths of thefirst and second peripheral zones are equal.
 6. The method as recited inclaim 1 wherein widths of the first and second peripheral zones aredifferent.
 7. The method as recited in claim 1 further comprisingsimultaneously melting and vaporizing the first and second oppositeprincipal faces as the first and second principal are undergoing thesimultaneously melting and vaporizing step.
 8. The method as recited inclaim 1 wherein the intermetallic alloy layers remain in the first andsecond peripheral zones after the respective first and second metalalloy layers are removed.
 9. The method as recited in claim 1 whereinthe first and second steel substrates have different compositions. 10.The method as recited in claim 1 wherein the first and secondpre-coatings on the first and second steel sheets have differentthicknesses.
 11. The method as recited in claim 1 wherein the first andsecond metal alloy layers include, with the percentages expressed byweight, from 8 to 11% silicon, from 2 to 4% iron, and the a balance ofthe composition including aluminum and unavoidable impurities.
 12. Themethod as recited in claim 2 wherein the gap is greater than 0.02 mm.13. The method as recited in claim 2 wherein the gap is greater than0.04 mm.
 14. The method as recited in claim 1 wherein ablation rippleson the first and second peripheral zones align.
 15. The method asrecited in claim 1 wherein the simultaneously melting and vaporizing ofthe first and second metal alloy layers fully removes the first andsecond metal alloy layers in the first and second peripheral zones. 16.The method as recited in claim 15 wherein the simultaneously melting andvaporizing of the first and second metal alloy layers at least partiallyremoves the first and second intermetallic layers in the first andsecond peripheral zones.
 17. The method as recited in claim 16 whereinthe simultaneously melting and vaporizing of the first and second metalalloy layers removes at least 50% of the first and second intermetalliclayers in the first and second peripheral zones.
 18. A method forfabricating a welded blank comprising the steps of: preparing the firstand second sheets by the method as recited in claim 1; and welding thefirst sheet and the second sheet in the first and second peripheralzones, along a plane defined by the median plane.
 19. The method asrecited in claim 18 wherein the welding is performed at a location lessthan one minute after the melting and vaporizing at a same location onthe first sheet and the second sheet.
 20. The method as recited in claim18 wherein the welding is carried out by at least one laser beam. 21.The method as recited in claim 18 wherein the welding is carried outsimultaneously by two laser beams, a first of the two laser beamswelding on a side of the first and second principal faces, and a secondof the two laser beams welding of a further side of the first and secondopposite principal faces.
 22. The method as recited in claim 18 whereinthe melting and vaporizing is carried out by a laser beam, and the laserbeam and a welding device for the welding are combined in a single pieceof equipment, a relative speed of displacement of the laser beam and thewelding device in relation to that of the first sheet and the secondsheet being identical.
 23. The method as recited claim 18 wherein themelting and vaporizing is carried out by a laser beam, and a weldingdevice performs the welding, a maximum distance between impacts of thelaser beam and the welding device being less than or equal to 2 m. 24.The method as recited in claim 18 wherein the welding is carried outusing simultaneously at least one laser beam and one filler rod.
 25. Themethod as recited in claim 18 further comprising tracking the medianplane and recording coordinates (x-y) defining a location of the medianplane at an instant t, the coordinates (x-y) being used to guide thewelding.
 26. The method as recited in claim 18 wherein the melting andvaporizing step is guided by a first tracking of the median plane andthe welding is guided by a second separate tracking of the median plane.27. The method as recited in claim 18 further comprising clamping thefirst and second sheets during the melting and vaporizing step, theclamping being kept constant until the welding and at least during thewelding.
 28. A method for fabricating a press-hardened part from awelded blank, comprising the following steps: heating the welded blankfabricated according to claim 18 to confer a partially or totallyaustenitic structure on the first and second steel substrates; hotforming the welded blank to obtain a part; and cooling the part at arate sufficient to form at least martensite or bainite in the first andsecond steel substrates.
 29. The method as recited in claim 28 wherein aweld formed by the welding in the part after the cooling is free ofFe—Al intermetallic compounds.
 30. A hot stamped part made according tothe method of claim 28.